Re: [Xen-devel] [PATCH v6 04/12] xen: add basic hypervisor filesystem support

[Jürgen Groß <jgross@xxxxxxxx>]

On 04.03.20 14:03, Jan Beulich wrote:
> On 04.03.2020 13:00, Jürgen Groß wrote:
> > On 03.03.20 17:59, Jan Beulich wrote:
> > > On 26.02.2020 13:46, Juergen Gross wrote:
> > > > --- /dev/null
> > > > +++ b/xen/common/hypfs.c
> > > > @@ -0,0 +1,349 @@
> > > > +/******************************************************************************
> > > > + *
> > > > + * hypfs.c
> > > > + *
> > > > + * Simple sysfs-like file system for the hypervisor.
> > > > + */
> > > > +
> > > > +#include <xen/err.h>
> > > > +#include <xen/guest_access.h>
> > > > +#include <xen/hypercall.h>
> > > > +#include <xen/hypfs.h>
> > > > +#include <xen/lib.h>
> > > > +#include <xen/rwlock.h>
> > > > +#include <public/hypfs.h>
> > > > +
> > > > +#ifdef CONFIG_COMPAT
> > > > +#include <compat/hypfs.h>
> > > > +CHECK_hypfs_direntry;
> > > > +#undef CHECK_hypfs_direntry
> > > > +#define CHECK_hypfs_direntry struct xen_hypfs_direntry
> > > I'm struggling to see why you need this #undef and #define.
> > Without those I get:
> >
> > In file included from /home/gross/xen/unstable/xen/include/compat/xen.h:3:0,
> >                    from /home/gross/xen/unstable/xen/include/xen/shared.h:6,
> >                    from /home/gross/xen/unstable/xen/include/xen/sched.h:8,
> >                    from /home/gross/xen/unstable/xen/include/asm/paging.h:29,
> >                    from
> > /home/gross/xen/unstable/xen/include/asm/guest_access.h:1,
> >                    from
> > /home/gross/xen/unstable/xen/include/xen/guest_access.h:1,
> >                    from hypfs.c:9:
> > /home/gross/xen/unstable/xen/include/xen/compat.h:134:32: error:
> > redefinition of ‘__checkFstruct_hypfs_direntry__flags’
> >    #define CHECK_NAME_(k, n, tag) __check ## tag ## k ## _ ## n
> >                                   ^
> > /home/gross/xen/unstable/xen/include/xen/compat.h:166:34: note: in
> > definition of macro ‘CHECK_FIELD_COMMON_’
> >    static inline int __maybe_unused name(k xen_ ## n *x, k compat_ ## n *c) \
> >                                     ^~~~
> > /home/gross/xen/unstable/xen/include/xen/compat.h:176:28: note: in
> > expansion of macro ‘CHECK_NAME_’
> >        CHECK_FIELD_COMMON_(k, CHECK_NAME_(k, n ## __ ## f, F), n, f)
> >                               ^~~~~~~~~~~
> > /home/gross/xen/unstable/xen/include/compat/xlat.h:775:5: note: in
> > expansion of macro ‘CHECK_FIELD_’
> >        CHECK_FIELD_(struct, hypfs_direntry, flags); \
> >        ^~~~~~~~~~~~
> > /home/gross/xen/unstable/xen/include/compat/xlat.h:782:5: note: in
> > expansion of macro ‘CHECK_hypfs_direntry’
> >        CHECK_hypfs_direntry; \
> >        ^~~~~~~~~~~~~~~~~~~~
> > hypfs.c:19:1: note: in expansion of macro ‘CHECK_hypfs_dirlistentry’
> >    CHECK_hypfs_dirlistentry;
> >    ^~~~~~~~~~~~~~~~~~~~~~~~
> > /home/gross/xen/unstable/xen/include/xen/compat.h:134:32: note: previous
> > definition of ‘__checkFstruct_hypfs_direntry__flags’ was here
> >    #define CHECK_NAME_(k, n, tag) __check ## tag ## k ## _ ## n
> >                                   ^
> > /home/gross/xen/unstable/xen/include/xen/compat.h:166:34: note: in
> > definition of macro ‘CHECK_FIELD_COMMON_’
> >    static inline int __maybe_unused name(k xen_ ## n *x, k compat_ ## n *c) \
> >                                     ^~~~
> > /home/gross/xen/unstable/xen/include/xen/compat.h:176:28: note: in
> > expansion of macro ‘CHECK_NAME_’
> >        CHECK_FIELD_COMMON_(k, CHECK_NAME_(k, n ## __ ## f, F), n, f)
> >                               ^~~~~~~~~~~
> > /home/gross/xen/unstable/xen/include/compat/xlat.h:775:5: note: in
> > expansion of macro ‘CHECK_FIELD_’
> >        CHECK_FIELD_(struct, hypfs_direntry, flags); \
> >        ^~~~~~~~~~~~
> > hypfs.c:18:1: note: in expansion of macro ‘CHECK_hypfs_direntry’
> >    CHECK_hypfs_direntry;
> Which suggests to me that the explicit CHECK_hypfs_direntry invocation
> is unneeded, as it's getting verified as part of the invocation of
> CHECK_hypfs_dirlistentry.
Ah, right. This is working. Will change.

> > > > +int hypfs_write_leaf(struct hypfs_entry_leaf *leaf,
> > > > +                     XEN_GUEST_HANDLE_PARAM(void) uaddr, unsigned long ulen)
> > > > +{
> > > > +    char *buf;
> > > > +    int ret;
> > > > +
> > > > +    if ( ulen > leaf->e.size )
> > > > +        return -ENOSPC;
> > > > +
> > > > +    if ( leaf->e.type != XEN_HYPFS_TYPE_STRING &&
> > > > +         leaf->e.type != XEN_HYPFS_TYPE_BLOB && ulen != leaf->e.size )
> > > > +        return -EDOM;
> > > Why the exception of string and blob? My concern about the
> > > meaning of a partially written entry (without its size having
> > > changed) remains.
> > It is perfectly valid to write a shorter string into a character
> > array. I could drop the blob here, but in the end I think allowing
> > for a blob to change the size should be fine.
> But shouldn't this then also adjust the recorded size?
No, this is the max size of the buffer (you can have a look at patch 9
where the size is set to the provided space for custom and string
parameters).

> > > > +    buf = xmalloc_array(char, ulen);
> > > > +    if ( !buf )
> > > > +        return -ENOMEM;
> > > > +
> > > > +    ret = -EFAULT;
> > > > +    if ( copy_from_guest(buf, uaddr, ulen) )
> > > > +        goto out;
> > > > +
> > > > +    ret = -EINVAL;
> > > > +    if ( leaf->e.type == XEN_HYPFS_TYPE_STRING && !memchr(buf, 0, ulen) )
> > > This should also use the != buf + ulen - 1 form imo.
> > I'm fine to change that, but should the hypervisor really refuse to
> > accept a larger buffer?
> To avoid ambiguity I'd prefer if the requirement was that the
> caller specify the length of the string (plus the nul char)
> rather than the size of any buffer it might be using.
Okay, I don't mind changing it then.


Juergen

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